For the initial determination of a laser energy of a refractive laser systems usually a disposable energy calibration tool is used. This calibration tool consists of a glued foil on top of a coloured base plate. A laser system with nominal fluence level (expressed in mJ/cm2) will penetrate the foil with a specified number of pulses having two target positions on the tool. To test a laser system the number of laser pulses necessary to penetrate the foil is counted and the user determines when the foil is ablated, i.e., the coloured base plate is visible.
Modifications of such base plates are known, e.g., from U.S. Pat. No. 5,464,960 to Deborah K. Hall as laser calibration device. The laser calibration device for calibrating surgical lasers is formed by superimposition of thin-films of alternating colours. After ablation by a laser beam, the resulting spherical cavity appears as a pattern of nested circles whose concentricity and spacing reflect the alignment and intensity of the laser beam. These patterns can be visually or instrumentally analyzed to determine the proper setting of the laser. A monolayer or multi-layer thin film is used to determine not only the ablative power of a laser beam, but also the variation of the ablative power over the full breadth of the beam by observing the area impinged by the beam between successive laser pulses.
The determined number of shots necessary to ablate the foil and thus causing a colour change observed by a user may vary depending on the users subjective perception.
U.S. Pat. No. 7,211,078 relates to a device for monitoring the energy and/or position of a pulsed and scanned laser beam, wherein the pulsed laser beam is intermittently directed at a sensor. The measuring of laser energy and in particular the monitoring of laser energy is accomplished by using optical photodiodes, pyroelectric or thermoelectric sensors. In particular, the laser energy is monitored during operation by measuring a divided part of the laser beam. Alternatively, the entire undivided beam can be deflected onto a sensor.
An aspect of the invention is to improve laser energy measurement and monitoring. In particular, it is an object of the invention to improve the laser energy measurement and monitoring via sensors. A further aspect of the invention is to provide a more objective measurement utilizing a laser calibration tool in order to improve the measurement accuracy.